The accuracy of measurement of the optical loss in a light guide basically depends on the precision with which are measured the input and output losses of the guide. But in many practical cases the light guide allows access only to one of its ends: to the input. The optical loss at the output of the guide is then evaluated by the power of the radiation pulse scattered by the material of the guide, reflected by its end face and propagating in the return direction. In such cases an accurate measurement consists in determining the power of radiation introduced into the light guide.
Known in the art is a method of determining the optical loss in a light guide in reflected radiation, in which method from a radiation source a pulse is introduced into a splitter not connected to the guide to be investigated, the power of the pulse flowing through the splitter from the source to transducer is measured, the free end of the splitter is connected to the guide to be investigated and, with a proper time delay, follows a measurement of the power of the pulse which, being sent by the source through the splitter, has entered the guide and, reflected from the output end face of the guide, has come back by the same path in the guide to the transducer (JP, A, 59-12037). These measured quantities determine the optical loss in the guide. The power of the radiation pulse flowing through the splitter from the source to the transducer is measured in order to determine the radiating power introduced into the guide. When determining the optical loss in the guide, two rather weak signals are compared, with no need to measure the strong signal reflected from the free end of the splitter which might spoil the linearity of the transducer converting the power of this signal or even saturate the measuring system and, thereby, lead to its inefficiency, because the splitter is situated in a close proximity to the source and to the transducer.
However, in order to calculate, out of the power of the radiation pulse flowing through the splitter from source to transducer, the power of the radiation pulse introduced into the light guide under investigation, one must know all the optical characteristics of the splitter, whose measurement is in itself a complicated problem, whereas the inaccuracy of such a measurement would directly influence the accuracy of determination of the optical loss in the guide.
Known in the art is another method of measuring the optical loss in a light guide in reflected radiation on (JP, A, 58-100733), in which the free output end of a splitter is connected to a reflecting mirror, from a radiation source through a light guide simulator creating a stationary-mode distribution at the splitter input a pulse is introduced into the splitter, the power is measured in the radiation pulse reflected by the mirror at the output end of the splitter, the output end of the splitter is connected to the input end of the guide to be investigated, the output end of the guide is connected to said mirror and the power is measured in the radiation pulse which has entered the guide in question and, after reflection by the mirror, has followed the same path in its return to the transducer. The optical loss in the guide is evaluated from the relation between the powers measured. The use, at the output end of the splitter, of a high-reflecting mirror with a reflection factor near to unity allows to neglect parasitic reflections and the radiation flow inside the splitter, and thanks to it, to do without determining the optical characteristics of the splitter. But on the other hand, the use of mirrors leads to a high level of the compared powers, and this does not allow, at the same time and by one and the same instrument, to measure radiation reflected by the guide end face and radiation scattered by different sections of the guide, whereas such a simultaneous measuring is necessary if one analyses the distribution of optical parameters. This kind of analysis is used in the widely popular Optical Time Domain Reflectometry (OTDR).
Moreover, to realize such a method one must have access to both ends of the guide under study and this, as mentioned above, is not always possible. In addition, the use of mirrors complicates the introduction of such a method into practice.
Known is also a method of determining the optical loss in a fibre-optic guide in reflected radiation, which uses an idle light guide connected through a splitter with a source and a transducer of radiation and providing establishment of a stationary-mode distribution on the double (go and return) path in this guide, as well as excluding the non-linearity of conversion by said transducer of the power of the pulse reflected from the output end of the idle guide (M. A. Bukhshtab, "Izmerenie malykh opticheskikh poter", 1988, Energatomizdat (Leningrad), pp. 135-136). From the source a radiation pulse is introduced through the splitter into the idle light guide, and the transducer measures the P.sub.0 of the radiation pulse reflected by the output end face of the idle guide. The output end face of the idle guide is mated with the input end face of the guide under study leaving between them such an air gap that the total pulse of the radiation reflected from the output end face of the idle guide and of the radiation reflected from the input end face of the guide under study may not be considerably broader than the pulse of radiation reflected from the output end face of the idle guide. The mating is accomplished by means of special adjustable devices or couplers. Measured are the power P.sub.01 of the total radiation pulse reflected by the mated ends and the power P.sub.1 of the radiation pulse which has entered the guide under study from the idle guide through the air gap and, after reflection from the output end face of the guide under study, has returned by the same path to the transducer. In order to measure said power a time delay is introduced into the measuring system of the transducer, delay which provides that the transducer responds only to the pulses of radiation reflected from the output end face of the guide under study. In the first place, the measured power values permit to evaluate the optical loss in the joint between two light guides, and after that, the optical loss in the guide under study.
The use of an idle guide allows, the measurement results in hand, to eliminate completely the influence of the splitter optical characteristics and, besides, to compare powers of radiation pulses whose level is much lower than in case when mirrors are used. On the other hand, the total radiation pulse reflected by the mated ends of the guides has a power which is approximately double of the power of a single radiation pulse reflected by the output end face of the idle guide, and this fact may result in a non-linear conversion by the transducer of this total pulse or in a saturation of the measuring system used and, thereby, make impossible the measurement itself. In order that the transducer provide a linear conversion of the total power P.sub.01, one has to lower considerably the radiation power in the idle guide, but such a decrease will reduce the dynamic range of measurement of the optical loss in the guide under study. Furthermore, in this method it is the power of the radiation pulse reflected from the mated ends of light guides that is measured, while the power lost by the radiation when it passes the joint of two guides is determined by calculation which may bring systematic errors. This inaccuracy is due to the fact that the reflected signal maximum does not always coincide with the minimum loss in the joint, for example, when the end faces to be mated are parallel but the guides themselves are misaligned. Another drawback of the method is that the joint between the two guides, idle and subject to study, must always have an air gap, by which necessity the field of application of the method is limited. Otherwise, if said guides are mated without air gap, the total pulse of radiation reflected by the mated ends will disappear and measurements in reflected radiation by this method will become impossible.